The 3rd Generation Partnership Project Radio Access Network Long Term Evolution (hereinafter, referred to as LTE) (3GPP-LTE) employs orthogonal frequency division multiple access (OFDMA) for the downlink communication scheme and single carrier frequency division multiple access (SC-FDMA) for the uplink communication scheme. In addition, periodic sounding reference signals (P-SRS) are used in the uplink of LTE as reference signals for measuring uplink reception quality.
In LTE, in order to transmit a P-SRS from a terminal to a base station, an SRS transmission subframe which is common to all terminals (hereinafter, referred to as common SRS subframe) is configured. This common SRS subframe is defined by a combination of a predetermined periodicity and a subframe offset on a per-cell basis. In addition, the information on the common SRS subframe is broadcasted to terminals within the cell. For example, when the periodicity is equal to 10 subframes and the offset is 3, the third subframe in a frame (consisting of 10 subframes) is configured as a common SRS subframe. In a common SRS subframe, all the terminals within the cell stop transmission of data signals in the last SC-FDMA symbol of the subframe and use the period as the resources for transmission of the SRS (reference signals).
Meanwhile, subframes for SRS transmissions are individually configured for terminals by a higher layer (i.e., RRC layer higher than the physical layer) (hereinafter, referred to as individual SRS subframe). Each terminal transmits a P-SRS in the configured individual SRS subframe. In addition, parameters for SRS resources (hereinafter, may be referred to as “SRS resource parameters”) are configured and indicated to each terminal. The parameters for the SRS resources include the bandwidth, bandwidth position (or SRS frequency domain starting position), cyclic shift, and comb (corresponding to identification information on the subcarrier group) of the SRS, for example. The terminal transmits SRS using the resources specified by the indicated parameters. Additionally, SRS frequency-hopping may be configured.
In addition, the uplink of LTE supports only terminals including one antenna port. For example, as disclosed in NPL 1, transmission power PSRS(i) of the SRS in an i-th subframe is calculated according to the following Equation 1.[1]PSRS(i)=min{PCMAX,PSRS_OFFSET+10 log10(MSRS)+PO_PUSCH+α·PL+f(i)}   (Equation 1)
In Equation (1), PCMAX [dBm] indicates the maximum transmission power of a terminal, PSRS_OFFSET [dBm] indicates an offset value (parameter set from the base station) for the transmission power of PUSCH transmitted by the terminal, MSRS indicates the number of frequency resource blocks assigned to the P-SRS, PO_PUSCH [dBm] indicates an initial value (parameter set from the base station) of the transmission power of PUSCH, PL indicates a path loss level [dB] measured by the terminal, α indicates a weighting coefficient (parameter set from the base station) indicating the compensation ratio of the path loss (PL), and f(i) indicates a cumulative total value in the i-th subframe including past values of transmission power control (TPC) command (control value; for example, 3 dB, +1 dB, 0 dB, and −1 dB) subjected to closed loop control.
Similarly, transmission power PPUCCH(i) and PPUSCH(i) for the uplink control channel (PUCCH) and the uplink data signal (PUSCH) in the i-th subframe are calculated according to the following Equations 2 and 3, respectively.[2]PPUCCH(i)=min{PCMAX,PO_PUCCH+PL+h(nCQI,nHARQ)+ΔF_PUCCH(F)+g(i)}   (Equation 2)[3]PPUSCH(i)=min{PCMAX,10 log10(MPUSCH(i))+PO_PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}   (Equation 3)
In Equation 2, PO_PUCCH [dBm] indicates an initial value (parameter set from the base station) of the transmission power of PUCCH, h(nCQI, nHARQ) and ΔF_PUCCH(F) indicate parameters set according to the format type of PUCCH, the number of bits, and the like, and g(i) indicates a cumulative total value in the i-th subframe including the past values of TPC command subjected to closed loop control similar to f(i) of Equation 1. In addition, in Equation 3, MPUSCH(i) indicates the number of frequency resource blocks of PUSCH assigned in the i-th subframe, and PO_PUSCH(j) [dBm] and α(j) indicate an initial value of the transmission power of PUSCH and a weighting coefficient indicating the compensation ratio of path loss (PL), respectively, and are parameters set individually by the base station according to the type of semi-static assignment (j=0) and dynamic assignment (j=1). ΔTF(i) indicates an offset value that can be set according to the amount of control information when control information is transmitted using PUSCH.
In addition, in the uplink of LTE-Advanced, which is an advanced version of LTE (hereinafter, referred to as “LTE-A”), aperiodic SRS (hereinafter, referred to as A-SRS) is used in addition to P-SRS introduced in LTE. The transmission timing of A-SRS is controlled by trigger information (e.g., 1-bit information). This trigger information is transmitted to a terminal from a base station on a physical layer control channel (i.e., PDCCH) (e.g., see NPL 2). To put it more specifically, the terminal transmits A-SRS only upon request for A-SRS transmission made by the trigger information (i.e., A-SRS transmission request). In addition, there has been discussion on defining, as the transmission timing of A-SRS, the first common SRS subframe located after the fourth subframe from the subframe in which the trigger information has been transmitted. As described above, while terminals transmit P-SRS, periodically, terminals are allowed to transmit A-SRS in a concentrated manner within a short period only when uplink data transmissions occur in bursts, for example (e.g., see FIG. 1).
Moreover, LTE-A has control information formats for various types of data assignment indication. The control information formats in the downlink include: DCI format 1A for allocation of resource blocks consecutive in number (Virtual RBs or Physical RBs); DCI format 1, which allows allocation of RBs not consecutive in number (hereinafter, referred to as “non-contiguous bandwidth allocation”); DCI formats 2, 2A, 2B, and 2C for assigning a spatial-multiplexing MIMO transmission; a downlink assignment control information format for assigning a beam-forming transmission (“beam-forming assignment downlink format”: DCI format 1B); and a downlink assignment control information format for assigning a multi-user MIMO transmission (“multi-user MIMO assignment downlink format”: DCI format 1D). Meanwhile, the uplink assignment formats include DCI format 0 for assigning a single antenna port transmission and DCI format 4 for assigning an uplink spatial-multiplexing MIMO transmission. DCI format 4 is used for only terminals in which uplink spatial-multiplexing MIMO transmission is configured.
In addition, DCI format 0 and DCI format 1A are adjusted in size by padding so that each format consists of the same number of bits. DCI format 0 and DCI format 1A are also called DCI format 0/1A in some cases. DCI formats 1, 2, 2A, 2B, 2C, 1B and 1D are used in accordance with downlink transmission modes configured in each terminal (i.e., non-contiguous bandwidth allocation, spatial-multiplexing MIMO transmission, beam-forming transmission and multi-user MIMO transmission) and are formats to be configured in each terminal. Meanwhile, DCI format 0/1A can be used independently of the transmission modes and thus can be used for terminals in any transmission mode, i.e., DCI format 0/1A is a format commonly usable in all terminals. In addition, when DCI format 0/1A is used, single-antenna transmission or transmit diversity is used as the default transmission mode.
Terminals receive DCI format 0/1A and the DCI formats that are dependent on the downlink transmission modes. In addition, terminals in which uplink spatial-multiplexing MIMO transmission is configured receive DCI format 4 in addition to the DCI formats mentioned above.
In this respect, using DCI format 0 for indicating the trigger information for A-SRS has been discussed. DCI format 0 is a control information format used in indicating uplink data (PUSCH) assignment. The field for indicating the trigger for A-SRS is added to DCI format 0 in addition to RB indication field, MCS indication field, HARQ information indication field, transmission power control command indication field, and terminal ID field. It should be noted that A-SRS and P-SRS can be used together or singly. In addition, parameters for SRS resources (e.g., transmission bandwidth, cyclic shift, and/or the like) are configured independently for A-SRS and P-SRS.
In addition, there is a heterogeneous network using a plurality of base stations having coverage areas different in size. The heterogeneous network is a network in which a macro base station that covers a large coverage area (called a “macrocell” or “Macro eNB” in some cases) and a pico base station that covers a small coverage area (called a “picocell” or “Low Power Node (LPN)” in some cases) are used together. A method has been discussed by which transfer control (handover) is easily realized using the signal of the physical layer by giving the same identification number (cell ID) as a macrocell to a picocell disposed in the coverage area of the macrocell in the heterogeneous network. In the operation of such a heterogeneous network, a method of selecting optimal transmission and reception points from a plurality of cells according to the propagation conditions between a terminal and each cell has been discussed (for example, refer to FIG. 2 and NPL 3). As a signal that can be a candidate as an index for selection of transmission and reception points, there is a reference signal (for example, P-SRS and A-SRS) for uplink channel quality measurement from a terminal toward a base station.